The present invention relates to transferring wafers in a vertical CVD (Chemical Vapor Deposition) diffusion apparatus.
Nowadays, the manufacturing process of semiconductor elements or chips includes a CVD diffusion treatment step for them. In the treatment step, a predetermined number of silicon wafers are heated in a CVD diffusion apparatus in order to diffuse and deposit some material chemically on the silicon wafers. According to the conventional technology of manufacturing semiconductor chips, in order to uniformly and homogeneously diffuse and deposit some material on the wafers, a plurality of dummy wafers are arranged respectively at the top and the bottom of a column extending, for example, in a vertical line as shown in FIG. 11, sandwiching a group of product wafers. In the groups of product wafers, single monitor wafers are inserted in the product wafers at positions separated by a predetermined vertical distance.
The technology of manufacturing semiconductor chips will be explained in detail with reference to FIG. 11 showing one example of the wafer arrangement. The wafer group 1 is supported or held by a boat 2 in the diffusion furnace in order to apply the CVD diffusion treatment step to the wafers. The wafers are arranged in a predetermined manner and inserted into the boat 2 and the boat 2 is charged to the diffusion furnace. In general, the whole interior area or space of the diffusion furnace fails to have a uniform or homogeneous thermal distribution and consequently the boat 2 to be placed in the interior of the diffusion furnace has a surplus space relative to the number of product wafers to enable it to hold, for example, one and a half times the number of product wafers. According to the conventional technology, the product wafers are applied or placed in the portion of the boat to be placed at a part of the diffusion furnace, which has a uniform thermal distribution or the product wafers are placed in the portion of the boat having an uniform thermal distribution when the number of wafers to be treated is small relative to the furnace interior.
Two dummy wafer groups S and 4, respectively consisting of a suitable number of dummy wafers 1b, are situated at the upper end and the bottom end of the wafer arrangement held in the boat 2. Between the dummy wafer groups 3 and 4, there are groups of product wafers 5, each including a predetermined number of product wafers 1a. Each monitor wafer 1c are inserted into the group of product wafers 5 at suitable positions. A monitor wafer 1c is also placed between the lowest product wafer group 5 and the dummy wafer group 4.
FIG. 12 shows a conventional wafer transfer mechanism for transferring untreated wafers charged, or loaded, in a cassette 7 to the boat 2 and recharging the treated wafers to a vacant cassette 7. The wafer transfer mechanism shown in FIG. 12 is adapted to transferring or treating wafers one by one and it is called a one-by-one type mechanism. As is apparent from the drawing, the mechanism has a handling unit 6, around which there are a predetermined number of cassettes 7, holding wafers 1, respectively situated along the same circumference of the handling unit 6. A transfer elevator 8 and a load.multidot.unload elevator 9 are placed near the handling unit 6. The boat receiving stand 10 of the transfer elevator 8 is adapted to ascend and descend along a generatrix of the cylindrical surface of vertical diffusion furnace 12 including the circumference thereof. Vertical diffusion furnace 12 is placed above the boat receiving stand 11 of the wafer load.multidot.unload elevator 9. A shifting unit 13 for shifting boats 2 is installed between the transfer elevator 8 and the load.multidot.unload elevator 9.
The handling unit 6 has a rotation arm 14 ascending and descending with rotation around the circumference and a wafer sucking chuck 15 mounted on the rotation arm 14 so as to move along the diametric of the circumference along the rotation arm 14. The wafer sucking chuck 15 sucks the wafers 1 loaded in the cassette 7 one by one and takes them out of the cassette 7 and the respective wafers are placed successively upon the top wafer of the wafer group in the boat 2 set on the elevator 8. The transfer elevator 8 follows the progress of the wafer transfer and descends step by step. When transfer of all the wafers 1 from the cassettes 7 to the boat 2 is completed, the boat 2 containing wafers is shifted from the transfer elevator 8 to the load.multidot.unload elevator 9 by the shifting unit 13, and the load.multidot.unload elevator 9 then charges the boat 2 to the diffusion furnace 12.
After a CVD diffusion treatment step to be carried out in the furnace to the boat 2 is completed the treated boat 2 is taken out from the diffusion furnace 12. The boat 2 is then shifted to the transfer elevator 8 by means of the shifting unit 13 and the wafers follow the reverse processing course by the handling unit 6, being reloaded to the cassette 7.
FIG. 13 depicts another type of wafer transfer mechanism in which bundles of wafers are transferred to the boat 2.
In detail, the wafer sucking chuck 15 provided in the conventional wafer transfer mechanism shown in FIG. 13 has a set of twenty five (25) sucking plates 16. All sucking plates 16 function at a time to suck all twenty five wafers, for example, respectively loaded in a cassette 7 and transfer them collectively to the boat 2.
It is understood in the art that these respective wafer transfer mechanisms of the one-by-one type and the collective type have some shortcomings as will be described.
In general, the number of dummy wafers placed at the top and the bottom of the wafer arrangement to be placed on the boat 2, and the number of monitor wafers and the place at which these monitor wafers are inserted into the group of product wafers change according to the specific treatment condition and the particular treatment specification of customers. The former type of wafer transfer mechanism transfers wafers one by one to the boat, so it is possible to be applied to any case or any arrangement of the wafers. However, the conventional mechanism of this type has the shortcomings of requiring many operations of the mechanism, a long time required for transferring wafers, resulting in a deteriorating of the efficiency of the whole mechanism. Also, the need for many operations increases the possibility of dust generation from and around the moving parts of the mechanism, deteriorating the quality of product wafers or semiconductor chips.
On the other hand, the latter type of wafer transfer mechanism has the merits of a short time for transferring the wafers and accordingly the efficiency of this type of mechanism is very good.
However, it is noted that the mechanism of this type handles and treats twenty five wafers collectively or in one lot and in consequence it is impossible to rearrange or change the wafer arrangement during the transferring step. As a result, it is always necessary to previously arrange or charge in accordance with a predetermined wafer arrangement including dummy wafers, monitor wafers, and product wafers to the cassette. According to the prior art, the wafers are manually charged to the cassette and the manual charging operation of various kinds of wafers to the predetermined or desired pattern of the wafer arrangement is apparently very complicated, inducing bad efficiency of the mechanism and error insertion of wafers or wrong arrangement of wafers.
Considering the real situation of the technology, the applicant of the present invention previously filed Japanese Utility Model Appln. No.62-130524, in which a wafer transfer mechanism of a partly bulk transfer type can effectively transfer five wafers at a time to a boat. According to the present invention, the characteristics or merits of the one-by-one type and the collective type of the wafer transfer mechanism are combined, so that it is possible to exhibit the merits of both of these the wafer transfer method, the wafer transfer mechanism, and the transfer control apparatus to provide good efficiency without human error in insertion or arrangement of wafers.